This study examined how the degree of within-person variation (or temporal fluctuation) in relationship quality over time was associated with well-being (psychological distress and life satisfaction). A national sample of 18- to 34-year-old men and women in unmarried, opposite-sex relationships completed six waves of surveys every 4 months (N = 748). Controlling for initial levels of and linear changes in relationship quality, greater temporal fluctuation in relationship quality over time was associated with increasing psychological distress and decreasing life satisfaction over time. Decreased confidence in one’s relationship partially mediated these associations. Moderation analyses revealed that the association between fluctuations in relationship quality and change in life satisfaction was stronger for women, participants cohabiting with their partners, and those with greater anxious attachment, whereas the association between fluctuations in relationship quality and change in psychological distress was stronger for people with greater avoidant attachment.